Soichiro Yoshimoto

Kumamoto University, Kumamoto, Kumamoto, Japan

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Publications (87)337.03 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: From the standpoint of cytochrome c electrochemistry, it is important to understand the relationship between the electron transfer reaction and alkyl chain length of pyridine-terminated self-assembled monolayers (SAMs) at the molecular level. In the present study, SAMs of 4,4′-(dithiodibutylene)dipyridine ((PyC4S)2) formed on a Au(111) surface were investigated in aqueous electrolyte solutions by cyclic voltammetry (CV), surface-enhanced infrared absorption spectroscopy (SEIRAS) and in situ scanning tunneling microscopy (STM). Structural changes in the PyC4S-SAM were driven by changes in the applied electrode potential or pH of the electrolyte solution. In 0.05 M HClO4, a structural change in PyC4S-SAM was drastically induced by the potential manipulation from the open circuit potential to 0.10 V versus a reversible hydrogen electrode (RHE). A high-resolution STM image showed that the PyC4S-SAMs formed a p(12 × √3 R−30°) adlattice at ~0.15 V versus RHE. A semi-squarely arranged adlattice was newly formed at 0 V versus RHE. The reductive peaks that appeared in the CV profile were thus attributed to be a phase transition in the PyC4S-SAM. By changing the electrolyte in the solution from HClO4 to KClO4, PyC4S-SAM having a p(12 × √3 R−30°) lattice was changed to a p(8 × √3 R−30°) adlattice in 0.05 M KClO4 at pH 5.5, indicating that rearrangement of PyC4S-SAM took place. This difference in the adlayer structure between acidic and neutral solutions resulted from the protonation and deprotonation of the pyridine moieties in PyC4S-SAMs. Electrochemical investigations on the electron transfer reaction of cytochrome c revealed that the alkyl chain length in a Py-terminated thiolate SAM plays an important role in the heterogeneous electron transfer rate constant, ko′, but not in the diffusion coefficient, D, of cytochrome c. These results show that the structure of PyC4S-SAM can be controlled by changing the applied electrode potential and pH of the electrolyte solution. In addition, the effect of alkyl chain length in the Py-terminated SAM on cytochrome c electrochemistry is clearly demonstrated. Long-range electron transfer kinetics is discussed on the basis of the Marcus theory.
    No preview · Article · Jan 2016 · The Journal of Physical Chemistry C
  • Hiroyuki Ueda · Katsuhiko Nishiyama · Soichiro Yoshimoto
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    ABSTRACT: The electrochemical behavior of cobaltocenium (Cc+) on a Au(111) electrode was investigated in five 1-alkyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([Cnmim][Tf2N], n = 2, 4, 6, 8, or 10) ionic liquids (ILs) in the temperature range from 293.15 to 343.15 K by cyclic voltammetry and chronoamperometry. The redox couple of Cc+ exhibited a clear reversible one-electron reaction in all the [Cnmim][Tf2N] ILs. The diffusion coefficients of Cc+ increased with an increase in the alkyl chain length of [Cnmim]+ and a decrease in the viscosity of the IL upon elevating the temperature. The viscosity of the IL plays an important role in determining the activation energy for the diffusion of Cc+. The obtained results suggested that the alkyl chain length of [Cnmim]+ affects the strength of the interaction between Cc+ and the surrounding ion species. The results also clarified that the equation proposed by Sutherland adequately describes the diffusion of Cc+ in ILs when the effect of the type of IL and the temperature on the product of the Stokes radius of Cc+ and the Sutherland coefficient is considered.
    No preview · Article · Jan 2016 · Physical Chemistry Chemical Physics
  • Teppei Kawamoto · Soichiro Yoshimoto
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    ABSTRACT: The adlayers of three metalloporphyrins-5,10,15,20-tetrakis(4-methoxyphenyl)porphyrin cobalt(II) (CoTMePP), 5,10,15,20-tetrakis(4-bromophenyl)porphyrin cobalt(II) (CoTBrPP), and 5,10,15,20-tetrakis(4-iodophenyl)porphyrin cobalt(II) (CoTIPP)-on Au(111) were investigated at the solid-liquid interface under electrochemical conditions. In situ scanning tunneling microscopy (STM) was employed to investigate the adlayer structures of CoTMePP, CoTBrPP, and CoTIPP in HClO4 solution. Highly ordered adlayers of the three metalloporphyrins were formed on the Au(111) electrode surface by simple immersion into benzene solutions containing each compound. The adlayer structure of the three cobalt porphyrin derivatives was influenced by the functional group on the phenyl moieties. In particular, a characteristic molecular assembly of CoTIPP molecules was found on Au(111) due to the I···I interactions between CoTIPP molecules. The adlattice constants increased in the order -OCH3 < -Br < -I in the phenyl groups. The in situ STM observations showed that the CoTMePP adlayer changed during positive potential manipulation in 0.1 M HClO4, whereas these adlayers were stable in the potential range from 0.90 to 0 V versus the reversible hydrogen electrode. A dependence upon the functional groups among the three CoTPP derivatives was clearly found in the adlattice constants and O2 reduction potentials, revealing that the 2-D molecular assembly and electrochemical activity for dioxygen reduction of the tetraphenylporphyrin derivatives can be tuned by introducing functional groups at the 4-positions of the phenyl moieties, especially iodine substituents.
    No preview · Article · Oct 2015 · Langmuir
  • Tomohiro Higashi · Teppei Kawamoto · Soichiro Yoshimoto · Takamasa Sagara
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    ABSTRACT: Two phase change processes of diphenyl viologen (dPhV) on a Au(111) electrode in KCl and KBr aqueous solutions were described using the results of voltammetric, electroreflectance (ER), and electrochemical scanning tunneling microscopic (EC-STM) measurements. Both processes exhibited sharp spikelike voltammetric responses. In KCl solution, the phase change at 0.30 V versus Ag/AgCl/saturated KCl was found to be a nonfaradaic order-disorder phase transition, from an ordered adlayer of dPhV dication (dPhV(2+)) with coadsorbed Cl- at more positive potentials than 0.30 V to a gaslike phase at less positive potentials. The faradaic reaction at -0.09 V was found to be the transition from the gaslike phase to a condensed monolayer of dPhV(center dot+). The EC-STM images of the condensed monolayer showed stripe patterns of rows of pi-pi stacked dPhV(center dot+). Almost the same set of two processes was observed in KBr solution but not in KF solution. In KF solution, although two voltammetric responses were observed, the peaks were small and broad, indicative of sluggish adsorption state changes of individual dPhV cations. Taken together, specific adsorption of coexistent anions is of critical importance for the occurrence of the sharp nonfaradaic phase transition.
    No preview · Article · Jan 2015 · The Journal of Physical Chemistry C
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    ABSTRACT: Adsorption and electrochemical reaction of graphen oxide (GO) at an Au surface was studied by surface enhanced infrared absorption spectroscopy (SEIRAS). GO film on the Au electrode is formed within 1 min. in 0.4 mg ml-1 of GO aqueous dispersion solution. Also, it was shown that, by applying −1.0 V vs Ag/AgCl, the epoxy group of GO was reduced in 0.4 mg ml-1 of GO and 0.1 M NaClO4 solution, and the reaction was irreversible within the potential region under the experimental conditions studied here. [DOI: 10.1380/ejssnt.2015.413]
    Preview · Article · Jan 2015 · e-Journal of Surface Science and Nanotechnology
  • Soichiro Yoshimoto · Satoi Yasunishi · Teppei Kawamoto
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    ABSTRACT: The structures of 2D molecular assemblies of platinum(II) octaethylporphyrin (PtOEP) on Au(111) and Au(100) surfaces were examined using in situ scanning tunneling microscopy (STM) in 0.05 M HClO4 solutions. The in situ STM observations revealed that the reconstructed rows of Au(111) and Au(100) were stabilized through the formation of highly ordered arrays by adsorption of PtOEP in a wide potential range of the electric double-layer region and that the PtOEP overlayer expanded the stable potential range for the reconstructed phase. Furthermore, the supramolecular assembly of C60 and C84 on a highly ordered PtOEP adlayer formed on Au(111) surfaces was investigated. STM images revealed site-selective supramolecular assembly of fullerenes, especially C84, which exhibited a low coverage on the PtOEP adlayer. C60 molecules exhibited a full coverage and formed highly ordered adlayers on PtOEP, whereas C84 exhibited disordered arrays. The results of this study showed that the strong π-electron donation by the PtOEP molecules stabilized the reconstructed rows of single-crystal Au planes and that the PtOEP adlayer acted as a strong electron-donating layer, aiding the formation of supramolecularly assembled C60 molecules.
    No preview · Article · Dec 2014 · The Journal of Physical Chemistry C
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    ABSTRACT: To achieve coordination programming at an electrode surface, the first functional layer is highly important. In this study, high-resolution electron energy-loss spectroscopy (HREELS) was used to investigate coronene monolayer adsorption on a Au(111) substrate at 300 K. In the spectra, only a band attributable to the out-of-plane vibration mode was observed, suggesting that coronene molecules adsorb on the substrate with a flat orientation. Furthermore, the HREEL spectra showed that the coronene monolayer was stable even after heating to 400 K.
    No preview · Article · Aug 2014 · Chemistry Letters
  • Hiroyuki Ueda · Katsuhiko Nishiyama · Soichiro Yoshimoto
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    ABSTRACT: Control of the redox states of highly charged C60 anions in two ionic liquids (ILs) was investigated using cyclic voltammetry and differential pulse voltammetry. Highly charged reduced states of C60 5− and C60 6− could be produced and detected at room temperature by using an electrochemical interface between a multilayered C60 adlayer on a Au(111) electrode and N-butyl-N-methylpyrrolidinium-based IL, whereas a tributylmethylammonium-based IL provided less than four redox waves of C60. The multiple redox states of C60 were precisely controlled by a step-by-step, one-electron redox process. The results of the present study suggest that both a wide potential window and the interaction between C60 anions and IL cations are important for controlling the multiple redox states of C60 at room temperature.
    No preview · Article · Jun 2014 · Electrochemistry Communications
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    ABSTRACT: The effect of annealing on 4-PySH and 3-PySH SAMs on Au(111) and Au(100) substrates was examined using high-resolution electron energy loss spectroscopy (HREELS) for the first time. At 300 K, the surfaces of 4- and 3-PySH SAMs on Au(111) were partially covered with artifacts. However, after annealing at the appropriate temperature, clear HREEL spectra of 4-PySH and 3-PySH SAM on Au(111) were recorded, and both the 4-PySH and 3-PySH molecules adsorbed in a flat orientation. In contrast, on Au(100), the HREEL spectra of 4-PySH and 3-PySH SAMs prepared at 300 K revealed that the pyridine rings are oriented almost perpendicular to the surface. Annealing at an appropriate temperature had a dramatic effect on the surface structure of the 4-PySH and 3-PySH SAMs: Both the 4- and 3-PySH molecules adsorbed in a flat orientation.
    No preview · Article · May 2014 · Electrochemistry -Tokyo-
  • Soichiro Yoshimoto · Hiroto Ogata · Katsuhiko Nishiyama
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    ABSTRACT: The electrodeposition of gold and platinum complexes at the electrochemical interfaces on the coronene-modified Au(111) surface was investigated. Scanning tunneling microscopy revealed the spontaneous deposition of gold and platinum complexes, suggesting that the coronene adlayer plays an important role in the electroreduction of these complexes.
    No preview · Article · Aug 2013 · Dalton Transactions
  • Soichiro Yoshimoto · Katsuhiko Nishiyama
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    ABSTRACT: Redox-active adlayers consisting of 4', 4 ''''-(1,4-phenylene) bis(2,2':6',2 ''-terpyridine) ligand and Co2+ and/or Fe2+, were prepared on a Au(111) surface by a stepwise coordination method. The obtained adlayers produced a well-defined, stable redox wave associated with Co2+ and/or Fe2+ coordinated to 4',4 ''''-(1,4-phenylene) bis(2,2':6',2 ''-terpyridine). It was found that the ability to coordinate with PTPy adlayer formed on Au(111) was clearly dependent on the metal ions.
    No preview · Article · Jun 2013 · ECS Transactions
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    Soichiro Yoshimoto · Kingo Itaya
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    ABSTRACT: We describe the history of electrochemical scanning tunneling microscopy (STM) and advances made in this field during the past 20 years. In situ STM allows one to monitor various electrode processes, such as the underpotential deposition of copper and silver ions; the specific adsorption of iodine and sulfate/bisulfate ions; electrochemical dissolution processes of silicon and gold single-crystal surfaces in electrolyte solutions; and the molecular assembly of metalloporphyrins, metallophthalocyanines, and fullerenes, at atomic and/or molecular resolution. Furthermore, a laser confocal microscope, combined with a differential interference contrast microscope, enables investigation of the dynamics of electrochemical processes at atomic resolution.
    Full-text · Article · Jun 2013 · Annual Review of Analytical Chemistry (2008)
  • Soichiro Yoshimoto
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    ABSTRACT: The stability and phase transitions of adlayers of two cobalt(ii) porphyrins, 5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(ii) (CoTPP) and 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine cobalt(ii) (CoOEP), formed on Au(100) were investigated under electrochemical conditions. In situ scanning tunneling microscopy (STM) was employed to investigate the structure of CoTPP and CoOEP adlayers in 0.1 M HClO4. The CoTPP and CoOEP adlayer structures were varied with the modification time and the concentration. The in situ STM observations showed that the underlying reconstructed atomic structure was lifted to a (1 × 1) atomic arrangement by either the adsorption of CoTPP/CoOEP during modification in a benzene solution or positive potential manipulation in 0.1 M HClO4. Ordered CoTPP arrays with two different hexagonal and square packing arrangements were found on an Au(100)-(1 × 1) surface, along with characteristic Au islands. The CoOEP molecules also formed a close-packed hexagonal structure on an Au(100)-(hex) surface; CoOEP molecules were arranged in a semi-square structure on the Au(100)-(1 × 1) surface by the lifting of reconstruction. The results of this study showed that the interaction between the cobalt porphyrins and the Au(100) substrate depended on the modification conditions and the electrochemical potential.
    No preview · Article · May 2013 · Physical Chemistry Chemical Physics
  • K. Nishiyama · A. Kumatabara · H. Ueda · S. Yoshimoto
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    ABSTRACT: Stable Au electrodes for surface enhanced infrared spectroscopy (SEIRAS) were prepared using H-terminated Si as substrates. The prepared electrodes were stable even in strong alkaline solutions and enabled us to measure SEIRA spectra repeatedly. By injection the solution containing 4-PySH from the outside of the SEIRAS cell, adsorbed processes were observed easily and precisely. After injecting the solution, vibration bands corresponding to 4-PySH were observed at ca. 5-10 min, 1-3 min, and 0-1 min in DCl, NaClO4, and KOD, respectively. Analyzing the SEIRA spectra, the desorption potential of 4-PyS SAM in an acidic solution was evaluated to be - 1.2 V vs. Ag/AgCl.
    No preview · Article · May 2013 · ECS Transactions
  • Hiroyuki Ueda · Katsuhiko Nishiyama · Soichiro Yoshimoto
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    ABSTRACT: Electrochemical behaviors of the bis(trifluoromethylsulfonyl)imide ([TFSI]−)-based ionic liquids consisting of 1-butyl-3-methylpyrrolidinium ([Py1,4]+), methyltributylammonium ([N1,4,4,4]+), and methyltrioctylammonium ([N1,8,8,8]+) cations were investigated by using low-index Au single crystal electrodes by cyclic voltammetry. The voltammetric feature was dependent on both the structure of the cation and the crystallographic orientation of Au. The electrochemical potential window of the [TFSI]−-based ILs was found to be only dependent upon the chemical structure of the cation. For each Au crystal plane, the electrochemical potential window of each IL was roughly estimated to be approximately 5.5 V, 5.6 V, and 5.8 V for [Py1,4][TFSI], [N1,4,4,4][TFSI] and [N1,8,8,8][TFSI], respectively.
    No preview · Article · Mar 2013 · ECS Transactions
  • Soichiro Yoshimoto · Katsuhiko Nishiyama
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    ABSTRACT: A redox-active adlayer consisting of cobalt ions and terpyridine ligands, 4′,4′′′′-(1,4-phenylene)bis(2,2′:6′,2′′-terpyridine), was prepared on a Au(111) surface by a stepwise coordination method. The obtained adlayer produced a well-defined, stable redox wave associated with cobalt ions coordinated to 4′,4′′′′-(1,4-phenylene)bis(2,2′:6′,2′′-terpyridine), as compared to a tetra-2-pyridinyl-pyrazine adlayer, for which no redox wave was observed. In situ scanning tunneling microscopy revealed a structural change in the redox-active adlayer consisting of 4′,4′′′′-(1,4-phenylene)bis(2,2′:6′,2′′-terpyridine) and cobalt ions. It was found that the ability of cobalt ions to coordinate on Au(111) was clearly dependent on the chemical structure of the ligand, suggesting that ligand coordination with metal ions on the Au surface is determined by the molecular orientation and configuration of the ligand when the ligand is adsorbed on a Au substrate.
    No preview · Article · Jan 2013 · Journal of Inorganic and Organometallic Polymers and Materials
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    ABSTRACT: The 2D molecular assemblies of chloride-coordinated mixed-valence diruthenium complexes, each possessing phenyl, naphtyl, or anthracenyl moieties, were examined on an Au(111) at electrochemical interface. In situ scanning tunneling microscopy images revealed a clear dependence of the molecular assembly on both the nature of the aryl functional groups and on the redox state of the dinuclear ruthenium complex, either chloride-coordinated RuII/RuIII or noncoordinated RuII/RuII. At potentials where the RuII/RuIII and RuII/RuII redox states were in equilibrium, two distinct redox states were clearly identified at the single-molecular level. We found that manipulating both the electrochemical potential and the aryl functional group substitution was important for controlling the 2D molecular assembly of a chloride-coordinated diruthenium complex on an Au(111) surface.
    No preview · Article · Dec 2012 · The Journal of Physical Chemistry
  • Katsuhiko Nishiyama · Yuta Ono · Isao Taniguchi · Soichiro Yoshimoto
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    ABSTRACT: Two-dimensional (2D) molecular assemblies of 4',4 ''''-(1,4-phenylene)bis(2,2':61,2 ''-terpyridine) (PTPy) with several metal ions were investigated in 0.1 M HClO4 by cyclic voltammetry and electrochemical scanning tunneling microscopy (EC-STM). An STM image revealed that a disordered adlayer was deposited when iron ions were used, whereas well-ordered PTPy and gold complex adlayers were clearly observed when the electrode potential was precisely controlled. We succeeded in the direct formation of electrochemically active adlayers and the determination of the dependence of the electrocatalytic activity of 02 on the metal ion present.
    No preview · Article · Oct 2012 · Chemistry Letters
  • Source
    Kingo Itaya · Soichiro Yoshimoto
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    ABSTRACT: In this review, we focus on history and advances made in the past 20 years in the field of electrochemical scanning tunneling microscopy (EC-STM). It is demonstrated that in situ STM makes it possible to monitor with atomic and/or molecular resolution a wide variety of electrode processes such as underpotential deposition of copper and silver ions, the specifically adsorption of iodine and sulfate/bisulfate ions, electrochemical dissolution process of silicon and gold single crystal surfaces in electrolyte solution, and molecular assembly of metalloporphyrins, metallophthalocyanines and fullerenes. Furthermore, laser confocal microscope combined with a differential interference contrast microscope (LCM-DIM) investigation allows us to investigate dynamics of electrochemical processes with the capability of atomic layer resolution. Expected final online publication date for the Annual Review of Analytical Chemistry Volume 6 is June 15, 2013. Please see http://www.annualreviews.org/catalog/pubdates.aspx for ...
    Full-text · Article · Aug 2012
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    ABSTRACT: The influence of the alkyl chain length of the bis(trifluoromethylsulfonyl)imide-based ionic liquids 1-ethyl-3-methylimidazolium, 1-butyl-3-methylimidazolium, 1-hexyl-3-methylimidazolium, 1-methyl-3-octylimidazolium, and 1-decyl-3-methylimidazolium cation on the potential window was investigated with low-index Au single crystal electrodes by cyclic voltammetry. The potential window of the electrical double-layer region was found to depend on the crystallographic orientation of Au, which indicated a difference in the strength of cation adsorption on the Au substrate. The potential window of the electrical double-layer region was also dependent upon the alkyl chain length of the imidazolium cation.
    No preview · Article · Jul 2012 · Electrochemistry Communications

Publication Stats

2k Citations
337.03 Total Impact Points

Institutions

  • 1997-2015
    • Kumamoto University
      • • Priority Organization for Innovation and Excellence
      • • Department of Applied Chemistry and Biochemistry
      • • Faculty of Engineering
      Kumamoto, Kumamoto, Japan
  • 2006-2011
    • National Institute of Advanced Industrial Science and Technology
      • Biomedical Research Institute
      Tsukuba, Ibaraki, Japan
  • 2010
    • Hokkaido University
      • Catalysis Research Center
      Sapporo, Hokkaidō, Japan
  • 2001-2009
    • Tohoku University
      • Department of Applied Chemistry
      Miyagi, Japan